We propose to study the relations between common potentially low-penetrance polymorphisms in candidate genes for breast cancer in three prospective cohorts; the Nurses' Health Studies I and II, and the Women's Health Study. Specific hypotheses relate variants in genes important to steroid hormone metabolism (sex-hormone binding globulin, CYP 19 and the progesterone receptor) with risk of breast cancer, and effect modification of the relation of postmenopausal hormone use with breast cancer by genotype. We will also examine polymorphisms in two genes involved in cell cycle control (the HER2 proto-oncogene, the TGFbeta receptor type I) that have been associated with breast cancer risk in other studies. Finally, we will seek to replicate interactions we observed in the current grant cycle between initiation of smoking at a young age and variants in the CYP1A1 gene, and first degree family history of breast cancer and longer (CAG)n repeats in the androgen receptor. We estimate we will accrue 1540 cases of breast cancer in the Nurses' Health Study I, 261 cases in the Nurses' Health Study II, and 1050 cases in the Women's Health Study. With this total of 2851 incident cases, we will have >90 percent power to detect relative risks of 1.5 or greater for the main effects of most of the genotypes of interest. We will also have substantial power to detect interactions between these genotypes and established breast cancer risk factors.